This section reviews two scenario literatures. One is the SRES, which reports
on the development of multiple GHG emissions baselines based on different future
world views, and the other is the post-SRES literature, which involves the quantification
of mitigation scenarios based on the new SRES baseline scenarios.

2.5.1 Special Report on Emissions Scenarios: Summary and Differences from
TAR

2.5.1.1 IPCC Emissions Scenarios and the SRES Process

First, the reference scenarios are reviewed, namely the SRES GHG emissions
scenarios. These are reference scenarios in the sense that they
describe future emissions in the absence of specific new policies to mitigate
climate change. The new scenarios are published as the Special Report on Emissions
Scenarios (SRES) by the IPCC (Nakicenovic et al., 2000).

A key feature of the SRES process was that different methodological approaches
and models were used to develop the scenarios. Another was that an open
process was used to develop the scenarios through which researchers and
other interest groups throughout the world could review and comment on the SRES
scenarios as they were being developed. The SRES also aimed at improving the
process of scenario development by extensively documenting the inputs and assumptions
of the SRES scenarios; by formulating narrative scenario storylines; by encouraging
a diversity of approaches and methods for deriving scenarios; by making the
scenarios from different groups more comparable, and by assessing their differences
and similarities; by expanding the range of economic-development pathways, including
a narrowing of the income gap between developing and industrially developed
countries; by incorporating the latest information on economic restructuring
throughout the world; and by examining different trends in and rates of technological
change.

2.5.1.2 SRES Approach to Scenario Development

The basic approach of the SRES writing team was to construct scenarios that
were both qualitative and quantitative. The process involved first the formulation
of the qualitative scenario characteristics in the form of narrative storylines
and then their quantification by six different modelling approaches. The qualitative
description gives background information about the global setting of the scenarios,
which can be used to assess the capability of society to adapt to and mitigate
climate change, and for linking the emission scenarios with DES issues. The
quantitative description of emission scenarios can be used as input to models
for computing the future extent of climate change, and for assessing strategies
to reduce emissions.

The relation between qualitative and quantitative scenarios can be characterized
in terms of Figure 2.10.

The SRES writing team developed four scenario families (see Box
2.5 for an explanation of terminology used in the SRES), because an even
number helps to avoid the impression that there is a central or
most likely case. The scenarios cover a wide range  but not
all possible futures. In particular, there are no global disaster
scenarios. None of the scenarios include new explicit climate policies.

Each family has a unifying theme in the form of a storyline or
narrative that describes future demographic, social, economic, technological,
and policy trends. Four storylines were developed by the whole writing team
that identified driving forces, key uncertainties, possible scenario families,
and their logic. Six global modelling teams then quantified the storylines.
The quantification consisted of first translating the storylines into a set
of quantitative assumptions about the driving forces of emissions (for example,
rates of change of population and size of the economy and rates of technological
change). Next, these assumptions were input to six integrated, global models
that computed the emissions of GHGs and sulphur dioxide (SO2). As
a result, a total of 40 scenarios were produced for the four storylines. The
large number of alternative scenarios showed that a single storyline could lead
to a large number of feasible emission pathways.

In all, six models were used to generate the 40 scenarios that comprise the
four scenario families. Six of these scenarios, which should be considered equally
sound, were chosen to illustrate the whole set of scenarios. They span a wide
range of uncertainty, as required by the SRES Terms of Reference. These encompass
four combinations of demographic change, social and economic development, and
broad technological developments, corresponding to the four families (A1, A2,
B1, B2), each with an illustrative marker scenario. Two of the scenario
groups of the A1 family (A1FI, A1T) explicitly explore energy technology developments,
alternative to the balanced A1B group, holding the other driving
forces constant, each with an illustrative scenario. Rapid growth leads to high
capital turnover rates, which means that early small differences among scenarios
can lead to a large divergence by 2100. Therefore, the A1 family, which has
the highest rates of technological change and economic development, was selected
to show this effect.

To provide a scientific foundation for the scenarios, the writing team extensively
reviewed and evaluated over 400 published scenarios. Results of the review were
published in the scientific literature (Alcamo and Nakicenovic, 1998), and made
available to the scientific community in the form of an Internet scenario database.
The background research by the six modelling teams for developing the 40 scenarios
was also published in the scientific literature (Nakicenovic, 2000).

Model: a formal representation of a system that allows quantification
of relevant system variables.Storyline: a narrative description of a scenario (or a family of
scenarios) highlighting the main scenario characteristics, relationships
between key driving forces, and the dynamics of the scenarios.Scenario: a description of a potential future, based on a clear
logic and a quantified storyline.Family: scenarios that have a similar demographic, societal, economic,
and technical-change storyline. Four scenario families comprise the SRES:
A1, A2, B1, and B2.Group: scenarios within a family that reflect a variation of the
storyline. The A1 scenario family includes three groups designated by
A1T, A1FI, and A1B that explore alternative structures of future energy
systems. The other three scenario families consist of one group each.Category: scenarios are grouped into four categories of cumulative
CO2 emissions between 1990 and 2100: low, mediumlow,
mediumhigh, and high emissions. Each category contains scenarios
with a range of different driving forces yet similar cumulative emissions.Marker: a scenario that was originally posted on the SRES website
to represent a given scenario family. A marker is not necessarily the
median or mean scenario.Illustrative: a scenario that is illustrative for each of the six
scenario groups reflected in the Summary for Policymakers of this report.
They include four revised scenario markers for the scenario
groups A1B, A2, B1, and B2, and two additional illustrative scenarios
for the A1FI and AIT groups. See also (Scenario) Groups and
(Scenario) Markers.Harmonized: harmonized scenarios within a family share common assumptions
for global population and GDP while fully harmonized scenarios are within
5% of the population projections specified for the respective marker scenario,
within 10% of the GDP and within 10% of the marker scenarios final
energy consumption.Standardized: emissions for 1990 and 2000 are indexed to have the
same values.Other scenarios: scenarios that are not harmonized.